The present invention relates to a sheet-holding device such as is used in an internal drum plotter. The invention also relates to a method for holding a sheet against a surface of such a device, and to an internal drum plotter including such a device.
Internal drum plotters are widely used for transferring visual and other data to recording material. In such plotters, a flexible sheet of light sensitive material such as film is applied to the inner surface of a generally circular cylinder or drum and an optical head located on the central axis of the drum reflects a modulated light beam onto the film. The exposed film is then removed from the drum and a new sheet is inserted. Usually, the light sensitve film is fed from a roll in an input cassette onto the drum and, after being exposed, is cut and removed from the drum into an output cassette.
A crucial aspect in the exposure of the film is that the focal plane of the film retain uniform geometrical precision in relation to the optical head. That is, the distance of the film from the head must be equal over the entire inner surface of the drum. This requires that the film closely and uniformly adhere to the inner surface of the drum.
Several methods are in use today to insure proper adherence of the film to the drum.
In one such method, the film is loaded onto the inner surface of the drum by a loading mechanism and removed from the drum by a discharging mechanism. A slight increase in the loading rate over the discharging rate causes the film to bow outwardly towards the inner surface of the drum. Final adherence is achieved by halting the discharging process slightly before cessation of the loading process.
In another method, a roller is used to feed and press the film onto the drum surface while a vacuum system is applied to the entire inner surface of the drum to hold the film in place.
A third method is described in U.S. Pat. No. 4,853,709, assigned to the applicants. In this method, the film is fed by a film driving roller, and tight engagement with the inner surface is provided by applying a compression force in the plane of the film. This force is attained by a film pressure roller which acts in concert with the film driving roller to feed or hold the film, depending on the position of the pressure roller, and by a film stop bar located at the opposite edge of the film. Final adherence is achieved by the film stop bar applying mechanical pressure on the film at one edge, thereby arresting the film's movement, while at the same time the film pressure roller, acting as a slipping clutch, propels the film forward an additional short distance before halting and securing the film in place.
Although the above methods bring about the adherence of the film to the inner surface of the drum, they suffer from a number of disadvantages. Firstly, the movement of the film suffers from unequal friction between the film and drum surfaces. Due to this unequal friction, it is difficult to keep the film parallel to the circumferential axis of the drum, requiring constant supervision of the loading and discharging processes. In addition, the use of various rollers can cause creases in the film distorting its dimensions. Finally, applying a vacuum along the entire inner surface of the drum causes the areas of the film held by the vacuum to adhere more closely to the drum than the in-between areas, resulting in an uneven adherence of the film to the drum surface.